1. Field of the Invention
This invention relates to a charging circuit that includes a disconnection detection unit for an external power supply and a power path control unit to control the supply of power to a system load and a battery. More specifically, to a charging circuit that includes an external power supply disconnection detection circuit that operates reliably without erroneous detection even when an external input voltage drops due to overload of the system, and without generating a reverse current when the external input voltage is decreased, and to a method of controlling the charging circuit.
2. Description of the Related Art
A charging circuit and devices which include the charging circuit generally mount a reverse current prevention circuit. In a reverse current prevention circuit, as disclosed in JP-3329168-B2, a resistor is connected in series between a power supply input and a battery, and voltages at both ends of the series resistor are compared to detect a reverse current to perform a halt control (disconnection detection).
In a reverse current prevention circuit disclosed in JP-2008-228416-A, when it is detected that the charging current falls below a predetermined value, a switch for preventing a reverse current is turned on and off in a predetermined constant cycle using an oscillator. When it is detected that the input voltage is decreased when the switch for preventing the reverse current is off, it is determined that a reverse current has occurred and a halt control operation (disconnection detection) is performed.
However, in the invention disclosed in JP-3329168-B2, to detect the reverse current, it is necessary to employ a resistor connected in series. Accordingly, a power loss at the resistor is definitely generated. When a resistor having a low resistance is used, a comparator having high precision is required. Further, in a charging circuit having a power path control system, there may he no voltage drop across the resistor under no system load conditions. Accordingly, there is a possibility of erroneous detection when only the voltage drop across the resistor is detected.
In the invention disclosed in JP-2008-228416-A, the decrease in the input voltage at the detection cycle of the reverse current is compared with a predetermined voltage value so that it is determined that it is in the reverse current condition when the input voltage is lower than the predetermined voltage value. Accordingly, when an unstable power supply such as a solar cell is used as the power source of the input voltage, there is a possibility that the input voltage cannot he detected correctly.